After achieving a direct measurement of forces between DNA double helices in solution last year, we have systematically investigated the determination of that force by the ionic species bound to the molecular surface. An especially exciting byproduct of our investigation was the realization that DNA in bacteriophage heads is under conditions similar to those under which we made our measurements. We have thus determined the long sought "DNA pressure" of packing the viral head. The fact that our measurements show qualitative differences from popular theories adds to the interest in our findings. Studies on bilayer membrane interaction have revealed the importance of these forces in bilayer deformation and uncovered conceptual difficulties associated with models of cell membrane fusion. In all these systems the paramount role of "hydration" forces, first identified by Parsegian and Rand, is evident. The primacy of hydration factors is emerging as a theme of molecular assembly in cellular and subcellular systems.